In a wavelength division multiplexing (WDM) optical transmission system, optical signals at different wavelengths are encoded with digital streams of information. These “wavelength channels” are combined together and transmitted through a series of spans of optical fiber. At a receiver end of a transmission link, the wavelength channels are separated, and each wavelength channel is individually detected.
While propagating through an optical fiber, light becomes attenuated. Yet some minimal level of optical power is required at the receiver end to decode information that has been encoded at the transmitter end. To boost optical signals propagating in an optical fiber, optical amplifiers are deployed throughout the transmission link. Optical amplifiers extend a maximum length of the link, in some instances, from a few hundred kilometers to several thousand kilometers, by amplifying optical signals to power levels close to the original levels of optical power.
A spectral gain profile is an important characteristic of an optical amplifier. It is desired to have a flat spectral gain profile, so that all wavelength channels are amplified similarly. Since an inherent gain profile of most optical amplifiers is not flat, gain flattening optical filters (GFFs) are often employed which attenuate gain peak or peaks of an optical amplifier, typically achieving a spectral flatness of approximately ±0.5 dB. However, the gain profile of many amplifiers, particularly an erbium doped fiber amplifier (EDFA), is dependent on temperature, so a good spectral flatness is only achievable in a narrow temperature range. To keep the gain profile flat at a wide range of ambient temperatures, the thermal dependence of an optical amplifier gain needs to be reduced or externally compensated.
One method to reduce thermally induced EDFA gain variations is to thermally stabilize the active optical fiber of EDFA, that is, the erbium doped fiber (EDF). By way of example, Pelard et al. in U.S. Pat. No. 6,535,329 disclose stabilizing a spool of EDF by heating the EDF spool to an elevated temperature and providing an optical feedback loop by measuring optical spectrum of optical signal amplified by the EDFA. Referring to FIG. 1, a Pelard apparatus 10 includes an EDF spool 33 in a package 30, an optical spectrum analyzer (OSA) 35, a computer 37, and a heating element 39. In operation, a portion of an output optical signal amplified in the EDF spool 33 is coupled to the OSA 35, which measures a spectrum of the amplified optical signal to obtain a “gain shape”. The computer 37 evaluates the spectral flatness of the gain shape, and provides a feedback control signal for the heating element 39 to heat the package 30, so as to preserve the spectral flatness.
Similarly, Flintham et al. in European Patent Application EP 1,246,322 disclose heating an EDF spool to reduce a gain deviation of an EDFA. Detrimentally, heating EDF to elevated temperatures may consume large amounts of energy and requires an extra space for thermal insulation of the EDF spool.
Chen et al. in U.S. Patent Application Publication 2002/0109907 disclose a dynamic compensator of thermally induced EDF gain variation including a fiber Bragg grating (FBG) having a thermally sensitive overcladding. When temperature of the FBG is tuned, an amplitude of the Bragg grating reflection peak changes. By properly selecting a spectral shape of the transmission peak in relation to the EDF gain profile, a certain degree of thermal stabilization of EDFA gain profile may be achieved. Unfortunately, FBG-based gain stabilizers are rather expensive, and require sophisticated active control.